Pamela Anderson running on a beach is one of those indelible images in American pop culture. So there were a lot of yuks when she announced earlier this fall that she'd be running in the New York City Marathon, which takes place this Sunday. Pam Anderson running—for 26.2 miles! And when she went on Ellen and confessed she actually despises running — and with three weeks to go had yet to run more than 12 miles — the former marathoner in me wondered: Does she have any idea what she's getting herself into?

When my friends found me after I'd completed the 2005 Boston Marathon (I ran unofficially, as a "bandit"), I was exhausted and disoriented. My armpits were chafed, my body ached, and I desperately needed to move my bowels. Salt was caked on my face from all of the dried sweat and shortly after stumbling into my friend's apartment on Newbury Street, I ate an entire Key Lime Pie and a bag of Doritos. I could tell that something was wrong, but I didn't know what. I also didn't know if the damage I'd inflicted on my body was permanent.

The next morning, I had a low-grade fever, diffuse muscle spasms, and I had to take a taxi to the medical school lecture hall. One of my classmates, a former college sprinter, had run the marathon on a whim (and without training) and told me he'd spent the evening pissing blood. Neither of us could navigate a flight of stairs unassisted. We were both a mess—as were most of my classmates who'd run the race—but we were a proud mess. Many said they couldn't wait to do it again. I was not one of them.

So what does a marathon do to your body?

This weekend, Pamela Anderson and thousands more will trek those 26.2 miles through the five boroughs of New York City and when they cross the finish line, they will have more in common with each other than sore muscles, chafed armpits, and a sense of accomplishment. Their bodies will desperately be trying to restore a physiological equilibrium that they've thrown into chaos during the race. During a marathon, the body undergoes significant changes to cope with the metabolic and physiological demands of running for such a long time. These include increases in the rate and depth of breathing, increasing the amount of blood that's pumped by the heart, redistribution of blood flow away from internal organs and toward muscle tissue, and changes to the circulating concentrations of various hormones. Crucial electrolytes—potassium, magnesium—may be severely disturbed during the event and in some cases, the abnormalities will be considered life-threatening.

Marathon runners routinely release molecules from the liver, heart, and skeletal muscles into the bloodstream that are usually only seen in patients with diseased organs2; from a biochemical perspective, many of the participants will resemble a corpse.

Perhaps the most alarming molecule that slips into the bloodstream during a race is something called the cardiac troponin enzyme3. The troponin molecule should be found in the blood in very low levels; any elevation can initially be cause for concern, and may herald the onset of a heart attack4. The tricky part is knowing when the troponin is just a transient finding, one that will go away with some rest and stuffing your face with Key Lime Pie and Doritos.

Can it give you a heart attack?

This Sunday, up to three-quarters of the marathon runners will have an abnormal elevation of troponin in their bloodstream5. The troponin molecule itself isn't doing harm; rather, it's a marker that the heart has sustained trauma. For the overwhelming majority of runners, however, it's important to know that these changes are transient and full recovery occurs within days, without any apparent long-term adverse consequences. Some runners, however, aren't so lucky. We've heard stories of people dropping dead during a marathon, and in many cases the patients are relatively young (mid-40s) and physically fit6. It wasn't always clear, however, if these marathon-related heart attacks were a common phenomenon or an exceedingly rare event that gets blown out of proportion because of the jarring imagery.

To examine this question, a large study searched for sudden death due to a heart problem (known as sudden cardiac death, or SCD) in 215,413 marathon runners who participated in a Marine Corps or a civilian marathon over a 19-year period7. As it turns out, sudden cardiac death occurred in only four individuals during or immediately following the marathon, an incidence of approximately one in 50,000, which is substantially lower than the annual risk of premature death in the general population. (Of the four who died suddenly, none had prior cardiac symptoms and two had completed several prior marathons.) Thankfully, and perhaps somewhat surprisingly, the incidence of SCD in marathon runners remains relatively low, possibly because so many are in such good shape to begin with.

What about losing too much salt?

The concern over the troponin enzyme seems rather straightforward; it should be in the heart, not in the blood stream. But other molecular abnormalities are more subtle yet potentially even more dangerous. The alarming scenario med students have drilled into their heads has to do with the hypothetical marathon runner who becomes increasingly delirious after a race. Young doctors are taught to assume that the runner has a profound disturbance of sodium, which actually doesn't come from consuming or losing too much sodium through perspiration; rather it comes from consuming an inappropriate quantity of water8. The fluid imbalance throws the sodium concentration out of whack, and through a complex mechanism can cause the brain to shrink or swell, particularly if too much water is consumed9.

The issue of sodium concentration in long-distance runners is potentially lethal and we've recently gotten a better sense of just how common it is. At the 2002 Boston Marathon, a sample of 488 runners approached randomly at race registration completed a survey prior to the race and, at the finish line, provided a blood sample, and completed a questionnaire detailing their fluid consumption and urine output during the race. Of these runners, 13 percent had abnormally low sodium concentrations (a condition known as hyponatremia) and three runners had what was considered critical hyponatremia, a condition that can land someone in the ICU. Extrapolating this to the 15,000 runners who finished that race, nearly 2,000 would have some degree of hyponatremia and nearly 100 would have critical hyponatremia. Sodium alteration is presumably why I felt so disoriented after the race.

What about the blood-pissing?

About that classmate of mine who said he peed blood after the marathon: It turns out he's not alone. The condition is called myoglobinuria and it develops as a consequence of muscle degradation brought on by extreme exertion. When muscle tissue is injured, it releases myoglobin into the bloodstream, much in the way that cardiac tissue released troponin. Myoglobin is eventually filtered through the kidneys and its presence in urine makes the liquid resemble tea. Depending on the temperature, up to 10 percent of marathon participants will be at risk for this condition.

Thousands will spend this Sunday throwing their internal organs into disarray. Thankfully, medical personnel will be on hand and most of the physical changes will fade. The psychological impact, however, will be much more permanent. Many will remember the race—and the charity they supported—for the rest of their lives. Others, like me, will never forget how grueling the experience was. I'm never doing it again. But I hope Ms. Anderson does. I'll be in Central Park on Sunday, cheering them all on.